two basic stages. the first is to buy/build an inverter to convert the variable voltage output to charge a battery. The second is to buy/build an inverter to convert the DC battery voltage to a suitable 3 phase AC supply for your pump. You will need a large battery, as your motor uses 5 X .75 KW ~ 3.8 KW, the inverter will use at least 4 KW, so I reckon more then 5 KW/Hrs. This would support your pump for one hour when the sun goes behind a cloud.
Frank